1. Field of the Invention
The present invention relates generally to methods and apparatus for producing lenticular sheets products, and more specifically, in an exemplary embodiment, to a manufacturing method for producing an extruded lenticular sheet having lenticules of substantially equal pitch.
2. Technical Background
In the commercial industry it is often desirable to impart visual effects such as three dimensionality or motion characteristics upon packages, labels, advertisements, entertainment devices, etc. Due to expense and ease of design, regular print was the pre-eminent and preferred form used on consumables and the like. Regular print is accomplished by printing two dimensional, non-movable information using words and graphics on packaging, labels, magazines, newspapers, brochures, fliers, posters, billboards, signs, etc. While some conventional print media are interesting, most are not. By way of example, the primary purpose for good print advertisements in packaging is to attract the attention of the reader or customer and convey the desired information. Unfortunately, many printed signs, advertisements and packages do not attract the attention of the desired audience or customers. As such, the use of products having lenticular effects has become increasingly popular.
Conventionally, lenticular lenses, or micro lenses, as used in imaging are typically elongated, lineal or dot patterned across an entire transparent sheet or web such that an image may be seen there through with the desired visual effect. The lenses can be either convex or concave in configuration. In addition, some lenses can be elliptical in shape. Other lens shapes or profiles are also possible (e.g., pyramidal, trapezoidal, parabolic, and the like). Desirably, the lenses have a pre-determined radius of curvature and a predetermined uniform or substantially equal pitch or repeat pattern. Lenticular lenses are thin, transparent lenses that are flat on one side and include a plurality of parallel, linear, side by side lenticules—elongate, convex ore concave lenses—on one side. Typically, an image is printed on the flat side to create a visual effect on the opposing side. The combination of the lenses and an image is referred to as a “lenticular assembly.”
In many cases, prior art lenticular assemblies and lenticular lenses are manufactured and produced in a continuous web with the lenticules being parallel to the longitudinal or latitudinal axis of the entire web. Manufacturing lenticular lenses is a highly specialized process. In conventional methods of manufacture, a thermoplastic resin material can be extruded onto a transparent pre-produced sheet or web (i.e., a film), and the lenticular lens pattern embossed into the resin by an embossing roll. More commonly, lenticular lenses are manufactured or produced using a machine or system which includes an extruder and a plurality of longitudinally stacked rollers that are used to move and support the sheet. It will be understood by those skilled in the art that conventional systems use three stack rollers, two of which are positioned one over the other, with the third roller disposed intermediate the first two rollers. In such systems, the first or upper roller and the second or lower roller usually have smooth outer surfaces. The intermediate roller is typically a lenticular pattern-forming device (e.g., an engraved cylinder) which includes a groove pattern on its outer surface. When a sheet or film is pressed against the groove pattern, a plurality of lenticular lenses or lenticules, which make up a lenticular pattern or array, are formed on a surface of the plastic sheet. In this way, a lenticular pattern is formed in the sheet or web that corresponds to the groove pattern. Accordingly, to meet the increasing needs of high quality lenticular lenses, it is necessary to design and fabricate an accurate, high quality lenticular pattern-forming device—cylinder.
Methods and apparatus for engraving cylinders with a negative or inverse lens patterns and other patterns are known in the art. In one such example, inverse lens patterns or shapes are engraved on a precision engraving and diamond turning machine into special metal cylinders and polished to a high luster. Conventionally, lenticular sheets have been produced by an extrusion/embossment process in the system described above using these cylinders. Typical thickness of extruded sheets or webs varies from about 0.007 inches to over 0.250 inches. Lenticular sheet widths up to 52 inches and more are capable and Lens Per Inch (LPI) patterns ranging from 10 to 300 are popular, however, any LPI pattern is possible. It will be understood by those skilled in the art that there are other methods of producing lenticular sheets using the same engraved cylinders, such as heat embossing and UV casting. Unfortunately, heat embossing does not always give an accurate method of reproducing the lens shape engraved in the cylinder. Casting a UV resin onto the engraved cylinder and curing the same with a strong UV light will give the most accurate method of reproducing the lens shape, but is a slow and expensive production method. The casting method is also limited to a thinner sheet (˜0.020 in. thick or less), unless it is laminated to a thicker clear sheet.
Disadvantageously, producing lenticular sheets or webs through the extrusion/embossment process on an extrusion machine does not have the accuracy of the casting method. It is commonly known that lenticular sheets and webs manufactured through an extrusion/embossment process will be heated to extreme temperatures to create a fluid suitable to form or mold. After the sheets or web exit the extrusion die, the thermoplastic resin will cool during the sheet forming process by coolant circulated through the rolls (extrusion nip) used to form the sheets. Afterwards, the thermoplastic sheet will continue to cool to ambient temperature of the manufacturing environment as it is pulled from the extrusion nip to end of the extrusion machine where cutting, sheeting or rolling is performed before packaging the product. As the plastic cools to room temperature, it will shrink in size based on its coefficient of thermal expansion/contraction along with the amount of tension applied during manufacturing. To provide proper cooling and produce a flat, commercially desirable sheet, an extrusion machine must be at least 40-100 ft in length. Disadvantageously, the pulling and cooling action over the length of the extrusion line produces a shrinkage, or ‘necking’, of the sheet or web.
The shrinkage or necking is shown by the unequal pitch of the plurality of lenticules across the sheet or web. As is known in the art, the overall pitch of a lenticular sheet is measured with a ‘pitch test’. A pitch test is a series of printed bars equally spaced across the width of the sheet. By way of example, one set of bars would be at a spacing equivalent to 20 LPI, the next row of bars at 20.01 LPI, and so on. By laying the clear lenticular sheet on the printed pitch test, one can determine which row of bars creates a solid color across the entire width of the image. This provides the average pitch that would be used to generate the interlaced image. If the overall image width is narrow, approximately 22 inches or less, this method has worked very well. As the width increases and the number of interlaced base images increases, the miss-registration of any interlaced image lines to the lenticular lines is more apparent. For manufactured lenticular sheets, the pitch at the center of the web more closely resembled the pitch of the engraved cylinder. The pitch thereafter incrementally decreased from the center lenticule to the outer lenticules at either web edge. Therefore, as you move from the center outward, the corresponding pitch of the each successive lenticule decreases. It has been found that the shrinkage is typically on the order of approximately one percent (1%) minimum and can be more. The amount of shrinkage depends on many factors such as pulling tension, cooling temperature, speed, through put per hour, web or sheet thickness, web or sheet width, make of extrusion/embossment machine, and length of extrusion line.
Another disadvantage of conventional systems, is that printing devices and printing presses as well as plate making equipment all operate in a linearly and uniform spacing fashion. The pixel layout on a computer is also configured in a uniform spacing arrangement. Programs like Photoshop®, CorelDraw®, etc. that are sometimes used to interlace images are likewise laid out in a continuous and uniform fashion. Various software programs designed to process or interlace the multiple images of 3-dimensional composite have likewise been designed in the same linearly and equal fashion. There have been attempts to correct the non-uniformity described above and increase in spacing across the extruded lenticular sheet by applying defined algorithms to the interlacing software. The result has been an undesirable increase in banding seen in the print. This is a condition that arises from the fact that each pixel on the computer is of equal size and the software is presenting unequal lines to the computer. The computer makes an adjustment which shows up as banding in the print.
In addition, cylinders are typically engraved at normal room temperatures and are run on the extruder at typically 160 to 180 degrees Fahrenheit. Understanding that the cylinders are metal and have a coefficient of heat expansion; that disadvantageously adds to non uniform spacing from the center to both sides of the cylinder.
Accordingly, there is a need for an improved method of manufacture for extruded lenticular sheet and web products. In one such solution, it would be desirable to have an engraved lenticular cylinder which includes spaced intervals that take into consideration the required shrinkage of an extrusion machine that will give a finished sheet of uniform and constant lens spacing. In another such solution, it would also be desirable to have an improved method of manufacture which includes the provision of a lenticular pattern-forming device (e.g., an engraved cylinder) which overcomes the noted shortcomings of conventional devices. In another such solution, it would be desirable to provide an improved engraved cylinder which has a reverse lens pattern thereon which is operable for producing a lenticular sheet with an array of lenticules having a substantially equal pitch. In such a solution, the engraved cylinder would have a reverse lens pattern comprised of a plurality of lenticules patterns, each with a variable pitch such that shrinkage caused by the extrusion process is compensated for.